Noble metals such as the platinum group are in general use as catalysts for automotive exhaust gas purification and organic chemical reactions, and fuel cell electrodes. They are also constituents of electronic parts such as ICs. The recovery of noble metals from used materials is important due to their rarity and expense.
This has hitherto been by the dissolution process. In this process, the metals are dissolved with the carriers in hot concentrated sulfuric acid or an oxidizing acid such as aqua regia, and the metals are separated from the solution by addition of a reducing agent or by electrolyzing the solution in low current density electrolytic cell. In the former case, the noble metals separate out in the solution, which in the latter case, they are deposited on the cathode.
The dissolution process requires a dangerously high oxidizing acid that is difficult to handle, and a large amount of heat to maintain the acid temperature. Additionally, aqua regia, long been used for noble metals dissolution, gives off NOx producing pollution treatment problems. Cyanides, such as sodium cyanide, are effective for noble metals dissolution, but need careful handling and adequate liquid waste treatment due to their high toxicity.
In addition, the dissolution process is uneconomical in noble metals recovery from noble metal catalysts, because it involves the treatment of a large amount of carrier containing solutions as well as noble metals. (Noble metal catalysts are composed of carriers such as silica and alumina and very small amounts of noble metals therein. Therefore, the volume ratio of noble metal to catalyst is very small: 1 g of noble metal to 1 liter in apparent volume of catalyst.) Moreover, at times a portion of noble metal is present as oxides, barely soluble in aqua regia. In such cases, it is necessary to reduce the noble metal oxides before dissolution, or perform the dissolution process in two steps, i.e., noble metal dissolution in acid and noble metal oxide dissolution in alkali (see Japanese Patent Laid-open No. 155333/1982).
On the other hand, several processes are proposed for electrolysis metal dissolution (see Japanese Patent Laid-open No. 37045/1976); however, nothing has been reported on the use of electrolysis to dissolve noble metals supported on granular carriers. In the case of noble metals, the resulting solution would be too dilute to handle economically. The addition of a reducing agent increases the amount of solution to be treated and therefore the operating cost, beyond practical consideration. The use of ordinary flat electrodes in the electrolytic cell creates a dilute solution that does not permit efficient recovery.